1. Field of the Invention
The present invention relates to the cleaning of printheads in computer-driven printers of the type generally known as inkjet printers. More particularly, the invention relates to such cleaning in printers employing a "wiper" which slidingly engages and wipes a nozzle orifice plate surface of a printhead to remove excess ink and accumulated debris to improve printhead performance and print quality.
2. Description of the Related Art
Ink-jet printing systems typically operate by ejecting ink from a plurality of small, closely-spaced nozzles located on the printhead. For proper functioning, an ink-jet printhead must be routinely serviced.
During printing, stray droplets of ink, dust, paper fibers and other debris can accumulate around the nozzles on the orifice plate surface and interfere with the trajectory of subsequently ejected ink droplets, thereby affecting print quality. To minimize this, the ink-jet printhead can be cleaned by intermittently wiping the orifice plate surface to remove the accumulated ink and debris.
During periods of inactivity, ink in the nozzles can dry or harden, plugging the nozzles. Thus, ink-jet printheads may be capped to maintain an appropriate environment around the nozzles and to postpone their clogging. This capping may be done automatically after a short period of inactivity, even during the middle of printing a page, if the printer is waiting for more data.
Inkjet printers generally have a printhead service station to which an inkjet printhead is moved by the carriage, and a cap which sealingly contacts the printhead is generally located at the service station. At the service station, the printhead (or multiple heads if such are used) are occasionally cleaned and, if necessary, primed with ink. For use in this cleaning function wipers are located at the service station. The service station can include a "sled" carrying these elements and others required to service the one or more printheads of the printer. This sled itself can be moved transversely to an axis of motion of the printhead carriage, for example in a vertical direction, so as to bring the caps or wipers for example into or out of contact with the printhead. Alternatively, a tumbler can be provided at the service station, and wipers, as well as caps, can be located on the tumbler. Rotation (and in some cases also vertical movement) of the tumbler effects wiping of the printhead, and/or alignment of one or more caps with one or more printheads positioned adjacent the tumbler at the service station.
To improve printing speed and the clarity and contrast of the printed image, recent advancements in the art have focused on improving the ink itself. For example, to provide faster, more waterfast printing with darker blacks and more vivid colors, improved pigment-based inks for inkjet applications have been developed. These pigment-based inks have a higher suspended solids content than earlier dye-based inks. Both types of ink dry quickly, which allows inkjet printing mechanisms to use plain paper. However, the combination of small nozzles and quick-drying ink leaves the printheads susceptible to clogging, in this case not only from the dried ink and minute dust particles or paper fibers, but also from the solids within the inks themselves. Further, this dried ink is more difficult to remove than previously used dye-based inks when dried. These characteristics compound the problems affecting print quality mentioned above.
Another characteristic of these pigment-based inks contributes to the nozzle clogging problem. The pigment-based inks use a dispersant to keep the pigment particles from flocculating. Unfortunately, the dispersant tends to form a tough film on the printhead orifice plate face as the ink "vehicle" or carrier component of the pigment-based ink evaporates. Besides the debris accumulated on the printhead face from ink over-spray, paper crashes and printer priming for example, this dispersant film also attracts and binds paper dust and other contaminants as well as solids from the ink itself. It has been recognized that this film, as well as ink residue and debris surrounding the printhead nozzles, is quite difficult to remove from the printhead.
Known cleaning systems used in printers of this type employ wipers which incorporate a blade formed of an elastomeric material such as a vinyl or EDPM. The wiper blade and a printhead are moved relative to one another so that the blade wipes accumulations from the critical area of the printhead incorporating the nozzle orifices. This system is not always fully effective even with older, dye-based inks. Some systems employ a second wiper formed of a soft absorbent material to further clean or "buff" the printhead. In other printhead servicing systems ink from the pen is ejected or drawn out and used to help lubricate the wiper and dissolve ink residue adhering to the printhead, with the goal of improving cleaning effectiveness. While this later scheme works well with some dye-based ink systems, it involves wasting ink that would otherwise be used for printing. Such a system is disclosed in commonly-owned U.S. Pat. No. 5,103,244 issued Apr. 7, 1992 to Gast, et al.
Furthermore, cleaning systems using ink drawn from the printhead do not work as well, generally speaking, with high-solids waterfast ink formulations. Reasons for this include the dried residue from such inks being more resistant to breakup and removal by mechanical forces applied by the wiper as mentioned above, and that the kinetics of redissolution are slow in these inks. These factors, for example, limit the effectiveness of this known cleaning process, and this is undesirable. Also, with this system more ink residue collects on the wiper, and some of these accumulations can be pushed back into the nozzles of the printhead which can at least temporarily cause one or more nozzles not to fire properly, degrading print quality.
It has been recognized that application of a fluid solvent or other treatment fluid to the printhead will mitigate the problem of dried ink by slowing the drying of ink or redissolving ink residue, rendering the printhead more easily cleanable by wiping. However, many problems associated with use of a treatment fluid have been identified.
Storage of the treatment fluid in adequate amounts for the life of the printer without leaking is problematic. For example leaks can occur due to tipping the printer and pressure differentials due to a change in temperature or altitude during shipment. Another problem recognized is application of treatment fluid to a printhead having undesirable accumulations of ink solids, dispersants, and other debris without contamination of the source of treatment fluid by such accumulations. It is desirable to maintain the means of applying treatment fluid and the treatment fluid itself in an uncontaminated state to provide consistent printhead cleaning over the life of the printer. Also, metering the amount of treatment fluid applied in wiping is recognized as important. Consistent optimal cleaning effectiveness as well as print quality can be compromised by application of too little or too much treatment fluid. Too little treatment fluid results in less effective residue removal allowing undesirable accumulation. Too much treatment fluid can result in one or more nozzles being at least temporarily disabled due to excess treatment fluid being pushed into the nozzle by the wiper, or drawn into the nozzle by negative pressure associated with operation.
In sum there are many problems, including identifying optimal ways of applying treatment fluid to enhance wiping effectiveness, that have been identified but not resolved in known cleaning systems. It is therefore recognized that an improved cleaning system, particularly for printers using pigment-based inks employing high solids content and dispersants, is needed to provide and maintain optimal functioning of thermal inkjet printheads.